Semiconductor device and method of manufacturing the same

ABSTRACT

The present invention provides a method of manufacturing a semiconductor device, comprising the steps of forming a gate insulating film, on a semiconductor substrate, forming a gate electrode containing a refractory metal layer on the gate insulation film, and heat-processing the semiconductor substrate in an atmosphere containing water vapor and hydrogen, to lessen a damage caused to a portion of the semiconductor substrate, which is located close to an end portion of the gate electrode. The heat-processing step is carried out while controlling a vapor pressure of a refractory metal oxo-acid generated on a surface of the high-melting metal layer.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device having a wiringlayer made of a metal or a metal laminate, and a method of manufacturingsuch a semiconductor device, and more specifically, to a semiconductorhaving a wiring layer containing a refractory metal, and a method ofmanufacturing such a device.

In the method of manufacturing an MOS (metal oxide semiconductor) typeintegrated circuit, some damages are caused to an end portion of a gatedue to RIE (reactive ion etching), ion injection, stress or the like. Inorder to lessen such damages, it is conventionally considered that anoxidization step should be provided. A gate electrode containingpolycrystalline silicon can be oxidized as the electrode is heated in anatmosphere containing dry oxygen or water vapor.

In the meantime, in order to realize a semiconductor device having afine size and operating at high speed, the development of a metal gateelectrode made of a single layer having a low resistance (to be called“metal gate electrode”) or a gate electrode having a laminate structureof metal and polycrystalline silicon (to be called polymetal gateelectrode) and the like, is presently being progressed.

However, in the oxidizing atmosphere described above, the metal is moreeasily oxidized than silicon due to the difference between them in theformation energy of an oxide. Thus, the oxidizing rate for metal is veryhigh, and therefore a gate electrode containing metal changes its shapesuch as the peeling off of the film, due to volume expansion.

In order to avoid this problem, a technique has been proposed (Jnp. Pat.Appln. KOKAI Publication No. 60-9166), for selectively oxidizing silicononly without oxidizing metal by heating an electrode in a mixture gas ofwater vapor serving as oxidizing agent, and hydrogen serving as reducingagent.

However, according to the researches made by the inventors of thepresent invention, it has been found that when a metal gate electrodemade of a refractory metal such as W or Mo, or a polymetal gateelectrode containing a refractory metal such as W or Mo, is subjected toa heat process carried out in an atmosphere containing hydrogen andwater vapor, the width of a metal electrode portion is reduced, or thelayer is thinned. In the case of a polymetal gate electrode, the crosssection of the electrode is decreased, and therefore the resistancevalue is increased. In the case of a metal gate electrode, not only theresistance value is increased, but also the effective gate length isdecreased, and therefore the electrical characteristics of the productwould not have appropriate values as designed.

Further, it has been found that if a heat process is carried out at atemperature of 800 to 900° C., not only the decrease in the width orthinning of the electrode occurs, but also whiskers are created on thesurface of a metal wiring. The whiskers, in some cases, have a length ofseveral hundred nano-meters. As a result, an interlayer insulation filmor the like cannot be deposited uniformly, and such non-uniformeddeposition may become a cause for short-circuiting or leakage ofcurrent.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a method ofmanufacturing a highly reliable semiconductor device, by suppressing thechange in the shape of the refractory metal layer.

Another object of the invention is to provide a highly reliablesemiconductor device by suppressing the change in the shape of therefractory metal layer which constitutes the gate electrode.

According to the present invention, there is provided a method ofmanufacturing a semiconductor device, comprising the steps of: forming aconductive layer including a refractory metal layer, on a semiconductorsubstrate; and heat-processing the semiconductor substrate having theconductive layer, in an atmosphere containing water vapor and hydrogen,wherein the heat-processing step is carried out while controlling thevapor pressure of oxo-acid of refractory metal generated on the surfaceof the refractory metal layer.

Further, according to the present invention, there is provided a methodof manufacturing a semiconductor device, comprising the steps of:forming a gate insulation film on a semiconductor substrate; forming agate electrode containing a refractory layer, on the gate insulationfilm; heat-processing the semiconductor substrate in an atmospherecontaining water vapor and hydrogen to lessen a damage caused to aportion of the semiconductor substrate, which is close to an end portionof the gate electrode, wherein the heat-processing step is carried outwhile controlling the vapor pressure of oxo-acid of refractory metalgenerated on the surface of the refractory metal layer.

Furthermore, according to the present invention, there is provided asemiconductor device comprising: a semiconductor substrate; a gateinsulation film formed on the semiconductor substrate; and a gateelectrode made of a laminate layer including a silicon layer formed onthe gate insulation film, and a refractory metal layer, wherein the endportion of the refractory metal layer projects sidewards from the endportion of the silicon layer, and a silicon oxide film is formed on aside surface of the silicon layer located underneath the end portion ofthe refractory layer.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments give below, serveto explain the principles of the invention.

FIG. 1 is a graph illustrating the temperature-dependency of the vaporpressure of oxo-acid of W, in the first example;

FIG. 2 is a graph illustrating the water vapor partial pressure/hydrogenpartial pressure—dependency of the vapor pressure of oxo-acid of W, inthe first example;

FIG. 3 is a graph illustrating the gas total flow-dependency of thevapor pressure of oxo-acid of W, in the first example;

FIGS. 4A to 4K are cross sectional views illustrating steps of themethod of manufacturing a semiconductor device, according to thesecond-example;

FIG. 5 is a photograph showing a gate structure corresponding to FIG.4J, taken under a TEM, which is obtained when selective oxidization iscarried out under a condition of the second example;

FIG. 6 is a photograph showing a gate structure corresponding to FIG.4J, taken under a TEM, which is obtained when selective oxidization iscarried out under a condition where the partial pressure of H₂O isincreased to a high level; and

FIGS. 7A to 7L are cross sectional views illustrating steps of themethod of manufacturing a semiconductor device, according to the thirdembodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the findings described below, obtainedin intensive studies made by the inventor(s) of the present invention,regarding the phenomenon that when a refractory metal layer isheat-processed in an atmosphere containing water vapor and hydrogen, thenarrowing and thinning of the layer occur, and further whiskers aregenerated on the surface of the refractory metal layer.

That is, a refractory metal is heat-processed in an atmospherecontaining water vapor and hydrogen, an oxo-acid of the refractory metalis formed. The oxo-acid has a vapor pressure higher than-that of anoxide of the refractory metal. In an extreme case, the refractory metallayer is narrowed and thinned so that the electrical resistance isincreased, or the oxo-acid is re-crystallized to generate whiskers onthe surface of the refractory metal layer, thereby causing thedeformation of the gate electrode.

It should be noted that under the conditions, 800° C. andP_(H20)/P_(H2)=0.75, the vapor pressure of WO₂, which is an oxide of W,is 8.3×10⁻¹⁸ atm, and the vapor pressure of WO₃ is 4.0×10⁻¹⁷ atm,whereas the vapor pressure of oxo-acid of W is 4.5×10⁻⁷ atm.

In consideration of the above facts, it was found that, in order toprevent the narrowing and thinning of the refractory metal layer and theformation of whiskers on the surface of the refractory layer, the amountof oxo-acid generated should be reduced, in other words, the vaporpressure of oxo-acid should be controlled.

As described above, the narrowing and thinning of the layer of the metalwiring can be avoided by controlling the vapor pressure of the oxo-acid,and therefore an increase in wiring resistance and degrading of shapecan be prevented. Therefore, it becomes possible to manufacture a highperformance semiconductor device.

It should be noted that the change in the shape of a refractory metallayer, indicated here is specified as a change caused by the narrowingand thinning of the layer, which occur as the refractory metalevaporates (that is, the generation of oxo-acid), and by the growth ofwhiskers on the surface of the refractory metal surface, which isinduced by the re-crystallization of the substances which were oncegasified.

Preferable aspects of the present invention will now be listed.

(1) The vapor pressure of oxo-acid of the refractory metal is controlledby controlling at least one parameter selected from the group consistingof controlling the ratio between a partial pressure of water vapor and apartial pressure of hydrogen, contained in an atmosphere, substratetemperature and gas flow.

(2) The heat process is carried out while sealing a gas containing watervapor and hydrogen in a reaction chamber.

(3) Prior to the heat processing step, an additional step of removingthe oxide layer formed on the surface of the refractory metal layer, isprovided.

(4) The vapor pressure of the oxo-acid of the refractory metal iscontrolled to be 1×10⁻⁶ atom or less.

(5) The ratio between the partial pressure of water vapor and thepartial pressure of hydrogen, contained in the atmosphere at atemperature of 800° C. is controlled to be 0.17 or less.

(6) During the heat process, the temperature of the substrate iscontrolled to be in a range between 700° C. and 950° C.

(7) The refractory metal is one selected from the group consisting oftungsten, molybdenum and titanium.

(8) The conductive layer is made of the refractory metal layer only.

(9) The conductive layer is made of a laminate member including apolycrystalline silicon layer and a refractory metal layer.

(10) A reaction preventing layer is provided between the polycrystallinesilicon layer and the refractory metal layer.

With the above-described structural items, the present inventionexhibits the following operational effects.

The researches made by the inventors have revealed the fact that thenarrowing and thinning of the refractory metal layer, which occur in theheat process carried out on the refractory metal, in a mixtureatmosphere of hydrogen, water vapor and some other diluting gas, occuras the metal oxo-acid of metal M having a high vapor pressure (that is,MH_(x)O_(y)) is generated. In the case where the vapor pressure of theoxo-acid generated is high, the amount of metal gasified is increased,and therefore the narrowing and thinning of the metal wiring are causedmore prominently.

Further, it has been found that whiskers formed on the surface of themetal layer are formed by the re-crystallization of the evaporated metaloxo-acid.

Therefore, in order to control the change in the shape of the metalwiring or electrode, made of the refractory metal, it suffices if theamount of metal gasified is reduced, that is, the vapor pressure ofmetal oxo-acid is decreased. More specifically, the partial pressure ofwater vapor, the partial pressure of hydrogen, the temperature and thegas flow should be adjusted so as to sufficiently reduce the vaporpressure of the metal oxo-acid.

Moreover, the researches conducted by the inventors of the presentinvention has revealed that in the case where the partial pressure ofwater vapor, the partial pressure of hydrogen, and the temperature arefixed, the amount of metal gasified increases as the total flow of theprocessing gas increases. Therefore, in order to control the evaporationof metal, it is. preferable that the flow of gas should be reducedduring the heat process.

Further, it has been found that in the case where the heat process iscarried out when an oxide layer is formed on the surface of the metalwiring, the metal oxo-acid is more easily generated. In order avoidthis, it is preferable that the oxide layer on the surface of the metalwiring should be removed prior to the heat process.

In the case where the size of the device is further reduced, theelectrode shape-dependency of the device property becomes moreprominent, and therefore the technique for suppressing the gasificationof a refractory metal, such as of the present invention becomes moreimportant.

In connection with the present invention, it should be noted that in thecase where the gate electrode is made of a laminate member consisting ofa silicon layer and a refractory metal layer, the silicon layer isoxidized and the size of the layer is decreased by the heat process inthe atmosphere containing water vapor and hydrogen. However, therefractory metal layer is not oxidized or the oxo-acid is not generated,and therefore the size of the metal layer does no change. Consequently,the end portion of the refractory metal layer projects sidewards fromthe end portion of the silicon layer, and a gate oxide film having astructure in which a silicon oxide film is obtained on the side surfaceof the silicon layer located underneath the end portion of therefractory metal layer.

The measurement of the end portion of the refractory metal layer, whichprojects sidewards from the end portion of the silicon layer, is usually1 to 10 nm.

Several examples of the present invention will now be described withreference to accompanying drawings.

EXAMPLE 1

FIGS. 1 and 2 illustrates, respectively, the temperature-dependency andpartial pressure-dependency of the vapor pressure of W oxo-acidgenerated by the oxidization which is carried out for lessen the damage,in the case where tungsten (W) is used as the material for the gateelectrode.

More specifically, FIG. 1 is a graph illustrating thetemperature-dependency of the vapor pressure of W oxo-acid in the casewhere the partial pressure of water vapor, P_(H2O) and the partialpressure of hydrogen, P_(H2), are fixed (P_(H2O)=0.08 atm, P_(H2)=0.04atm). As can be understood from FIG. 1, the vapor pressure of W oxo-acidis increased in an exponential function manner as the temperatureincreases.

Further, FIG. 2 is a graph illustrating the dependency of the vaporpressure of W oxo-acid in terms of the ratio between the partialpressures (P_(H2O)/P_(H2)) in the case where the temperature T is fixed.As can be understood from FIG. 2, as the partial pressure ratio becomeshigher, the vapor pressure of W oxo-acid is higher. Also, as the watervapor partial pressure, P_(H2O) becomes higher, the vapor pressure of Woxo-acid is higher.

According to the analysis of the inventors of the present invention, inorder to make the vapor pressure of W oxo-acid, WO₃H₂O, 1×10⁻⁵ atom orless, it is required that the heat process is carried out under theconditions of a temperature of 800° C. and (P_(H2O))⁴/(P_(H2))³<0.17.Under these conditions, the gasification of metal (the generation ofmetal oxo-acid) can be suppressed.

FIG. 3 is a graph illustrating the dependency of the W gasificationamount in terms of the total gas flow. As can be understood from FIG. 3,in the case where the water vapor partial pressure, P_(H2O), thehydrogen partial pressure, P_(H2) and the temperature T are fixed atconstant values, the gasification amount of the metal (corresponding tothe amount of decrease in the thickness of W) increases as the totalflow amount of the process gas increases. In consideration of this fact,in order to suppress the evaporation of the metal (that is, thegeneration of metal oxo-acid), it is effective to reduce the flow of theprocessing gas during the heat process.

As described above, when the water vapor partial pressure, P_(H2O), thehydrogen partial pressure, P_(H2) and the temperature T are adjusted andthe total flow of the processing gas are reduced, such that the vaporpressure of W oxo-acid becomes sufficiently low, the gasification of Wcan be effectively controlled. The narrowing and thinning of theelectrode, caused by the heat process can be controlled. In this manner,an electrode (wiring) having a predetermined shape and resistance can beobtained.

EXAMPLE 2

FIGS. 4A to 4K are cross sectional views showing the steps of themanufacture of a complimentary MOSFET (CMOSFET) according to the secondexample of the present invention.

First, a resist pattern (not shown) which covers a predetermined regionof a silicon substrate 10 is formed by a photolithography technique.With use of the resist pattern as a mask, B, Ga or In is ion-implantedto the silicon substrate 10. Then, after the resist pattern is removed,another resist pattern (not shown) which covers the surface of thatportion of the silicon substrate 10, to which the ions were injected inthe above-described step, is formed. With use of the resist pattern as amask, As, P or Sb is ion-implanted to the silicon substrate 10.Subsequently, a heat diffusion is carried out, and thereby a P-typeregion 11 and an N-type region 12, each having a depth of about 1 nm,are formed in the surface portion of the silicon substrate 10 as shownin FIG. 4A.

Next, as can be seen in FIG. 4B, an element isolation region 13 made ofa silicon oxide film having a thickness of about 600 nm is formed at aninterface section between the P-type region 11 and the N-type region 12of the silicon substrate 10, with use of, for example, LOCOS method.

Next, a protection oxide film (not shown) having a thickness of about 10nm is formed on the surfaces of the P-type region 11 and the N-typeregion 12. Then, another ion-implantation is carried out to match thethreshold value of the transistor, which will be formed later. Afterthat, the protection oxide film is removed, and as can be seen in FIG.4C, a gate oxide film 14 having a thickness of about several tens ofnanometers is formed on the surfaces of the P-type region 11 and theN-type region 12. It should be noted that in the cross sectional viewsof the steps from this onwards, the illustration of the substrate 10 isomitted.

Next, as can be seen in FIG. 4D, a polycrystalline silicon film 15having a thickness of about 100 nm is deposited on the entire surface.Then, a resist pattern (not shown) for covering a predetermined regionis formed by using the photolithography technique. With use of thisresist pattern as a mask, B,Ga or In is ion-implanted to thepolycrystalline silicon film 15. Similarly, in this case, with use ofthe resist pattern (not shown) as a mask, for covering the ion-implantedsection, As, P or Sb is ion-implanted to that portion of thepolycrystalline silicon film 15 which were not ion-implanted.

Next, as can be seen in FIG. 4E, a WSi_(x)N_(y) film 16 having athickness of about 1 nm is formed by carrying out a reactive sputteringin an atmosphere containing Ar and N₂ by using a Wsi_(x) target. TheWSi_(x)N_(y) film 16 serves as a reaction preventing layer forsuppressing the reaction between a W film, which will be later formed,and the polycrystalline silicon film 15, as well as a barrier layer forpreventing the diffusion of impurities in the polycrystalline siliconfilm 15 within the W film.

It should be noted that the formation of the WSi_(x)N_(y) film 16 may becarried out by use of a CVD method or the like, other than the methoddescribed above, or it is also possible that the film 16 may be formedby a heat process carried out at about 800° C., after forming a WN filmhaving a thickness of several nanometers by a reactive sputtering in anatmosphere containing Ar and N₂ using a W target.

Next, a tungsten (W) film 17 having a thickness of about 100 nm isformed by a sputtering in an Ar atmosphere using a W target, or the CVDmethod or the like. When the W film 17 is left in the atmosphere afterit is formed, a native oxide film 18 is formed in the surface portion ofthe W film 17.

Next, a reduction is carried out for about 30 minutes in a reducingatmosphere, so as to reduce the native oxide film 18 formed in thesurface portion of the W film 17. Usable examples of the reducing gasare H₂, CO and NH₃. Further, other than the reducing treatment, anetching operation which uses a chemical such as diluted sulfuric acid,CDE (chemical dry etching) or CMP (chemical mechanical polishing) or thelike may be used to remove the native oxide film 18.

If the resultant is left for 24 hours or more after the native oxidefilm 18 is removed, a native oxide film having a necessary amount forgrowing whiskers again, on the W film 17 is formed. In order to avoidthis, within several hours after the removal of the native oxide film, asilicon nitride film 19 having a thickness of about 250 nm is depositedon the entire surface by the CVD method or the like. Even though theprocessing temperature for this operation is about 800° C., whiskerswill never grow since the native oxide film on the W film 17 has beenremoved.

Next, as shown in FIG. 4G, a resist pattern 20 for forming a desiredgate electrode (wiring) is formed by using the photolithographytechnique. With use of the resist pattern 20 as a mask, the siliconnitride film 19 is patterned by the RIE method. Next, the resist pattern20 is removed by using an asher. With use of the silicon nitride film 19as a mask, the WSi_(x)N_(y) film 16 and the polycrystalline silicon film15 are etched by the RIE method, and thus a gate electrode (wiring) asshown in FIG. 4H is formed. After patterning, a native oxide film 21 isformed on the surface of the W film 17.

Next, as shown in FIG. 4I, a reduction is carried out at 600° C. forabout 30 minutes in a reducing atmosphere, so as to reduce the nativeoxide film 21 formed in the surface portion of the W film 17. Usableexamples of the reducing gas are H₂, CO and NH₃. Further, other than thereducing treatment, an etching operation which uses a chemical such asdiluted sulfuric acid or CDE or the like may be used to remove thenative oxide film 21. By removing the native oxide film 21,needle-shaped products are not generated on the side surface of the Wfilm 17 during a high-temperature heat process, which is carried outlater, and therefore the gate electrode maintains its original shape ofthe beginning of the process. Consequently, a low-resistance gateelectrode having a high reliability can be obtained.

Immediately after the removal of the native oxide film 21 by thereduction, an oxidization process is carried out for about 60 minutes at800° C. in an atmosphere containing N₂, H₂ and H₂O. This oxidizationstep is designed so that the W film 17 is not oxidized, but only thesurfaces of the P-type region 11 and N-type region 12, and thepolycrystalline silicon film 15 are oxidized. This step is calledselective oxidation process hereinafter.

In the selective oxidizing process, the bottom end portion of the gateelectrode (wiring) is oxidized, and a thick oxide film having a shape ofbird's beak, is formed. Consequently, the corner of the bottom endportion of the gate electrode (wiring) is rounded, and therefore theconcentration of an electrical field which occurs at the bottom endportion of the gate electrode (wiring) is loosened. Thus, theimprovement of the reliability and characteristics of the productelement can be achieved.

During the selective oxidizing process, the H₂O concentration, H₂concentration and heat-processing temperature are adjusted properly, andthus the generation of W oxo-acid, WH_(x)O_(y), having a high vaporpressure, which is formed on the surface of the W film 17 is suppressed.For example, in the case-where H₂ partial pressure is set at 2.5×10⁻²Torr and H₂O partial pressure is set at 2.5×10⁻³ Torr, the evaporationamount of the W film 17 (the amount of decrease in the thickness of thefilm) is suppressed to 1 nm or less.

With the employment of the conditions which suppresses the generation ofoxo-acid, the reducing rate of the cross section of the gate electrodehaving a width of 0.1 μm, for example, is 2% or less. The value of thereducing rate for the cross section of the gate electrode, which isdirectly connected to the gasification amount of the W film 17 in theselective oxidization process, is negligible as compared to theinfluences of the other factors which reduce the cross section of theelectrode, including the etching in the lateral direction by RIE whenprocessing the electrode.

The processing conditions vary depending upon the shape of the reactiontube and the gas stream condition, and in some cases, it is necessary topay attention to the adsorption or removal of H2O to or from thereaction tube, depending upon the material of the reaction furnace.

Next, as can be seen in FIG. 4J, a resist pattern (not shown) forcovering the P-type region 11 is formed with use of the photolithographytechnique. With use of the resist pattern as a mask, BF₂ ision-implanted under the conditions: 20 keV and about 7×10¹⁵ cm⁻², thusforming a P⁻-type region 23. In a similar manner, a resist pattern (notshown) for covering the N-type region 12 is formed. With use of thisresist pattern as a mask, As is ion-implanted under the conditions: 20keV and about 7×10¹⁵ cm⁻², thus forming a N⁻-type region 24.

Next, a silicon nitride film having a thickness of about 50 nm isdeposited by the CVD method, and then the silicon nitride film is etchedby the RIE method, thus forming a side-wall insulation film 25.

Next, as can be seen in FIG. 4K, a resist pattern (not shown) forcovering the P-type region 11 is formed with use of the photolithographytechnique. With use of the resist pattern as a mask, BF₂ ision-implanted under the conditions: 60 keV and about 7×10¹⁵ cm⁻², thusforming a P⁺-type region 26. In a similar manner, a resist pattern (notshown) for covering the N-type region 12 is formed. With use of thisresist pattern as a mask, As is ion-implanted under the conditions: 60keV and about 6×10¹⁵ cm⁻², thus forming a N⁺-type region 27. Lastly, anannealing is carried out so as to activate the P⁻-type region 23, theN⁻-type region 24, the P⁺-type region and the N⁺-type region 27.

Next, an interlayer insulation film and a wiring are formed by anordinary method, thus completing a CMOSFET.

According to this example, the vapor pressure of W oxo-acid issuppressed and therefore the narrowing and thinning of a W wiring whichconstitutes a gate electrode (wiring), and the growth of whiskers can besuppressed. Consequently, it is possible to form a gate electrode(wiring) of a desired shape and a designed resistance value.

FIG. 5 is a photograph taken under a microscope, showing a gatestructure corresponding to that of FIG. 4J in the case where selectiveoxidization is carried out under the conditions specified in thisexample. In the photograph of FIG. 5, a dark section in black is the Wfilm and a section underneath the black section is a polycrystallinesilicon film. A SiO₂ film is formed on a side of the polycrystallinesilicon film. The vicinity of the mid-portion of the SiO₂ film isoriginally a side surface of the polycrystalline silicon film, andfurther the position of this side surface is also the position of theside surface of the W film.

From the photograph shown in FIG. 5, it can be understood that the sidesurface of the polycrystalline silicon is oxidized by selectiveoxidization to become the SiO₂ film which grows in an inner and outersides of the polycrystalline silicon film.

Further, as can be seen in the photograph of FIG. 5, the mid-portion ofthe SiO₂ film coincides with the position of the side surface of the Wfilm. From this fact, it can be understood that the side surface of thepolycrystalline silicon film is oxidized by selective oxidization,whereas the W film is hardly oxidized. It should be noted that thenarrowed upper section of the side surface of the W film is resulted notby the oxidization but by the patterning of the W film by the RIE.

In contrast, FIG. 6 is a photograph taken under a microscope, showing agate structure corresponding to that of FIG. 4J in the case whereselective oxidization is carried out under the conditions specified inthis example except that the H₂O partial pressure is set to 3.0×10⁻²atom, and the H₂ partial pressure is set to 2.4×10⁻² atom, thus the H₂Opartial pressure being higher than the H₂ partial pressure. As can beseen in the photograph of FIG. 6, the vicinity of the mid-portion of theSiO₂ film does not coincides with the position of the side surface ofthe W film, but the position of the side surface of the W film islocated inward of the vicinity of the mid-portion of the SiO₂ film.Therefore, not only the side surface of the polycrystalline siliconfilm, but also the side surface of the W film are oxidized, and oxo-acidis created. Thus, the width of the film is reduced (narrowing). That is,under these conditions, the vapor pressure of W oxo-acid is 3.3×10⁻⁶atom, which is significantly higher than the vapor pressure (1.5×10⁻¹⁰atom) of W oxo-acid under the conditions of this example.

EXAMPLE 3

FIGS. 7A to 7L are cross sectional views showing the steps of themanufacture of a semiconductor device according to the third example ofthe present invention.

First, as shown in FIG. 7A, a resist pattern (not shown) which covers apredetermined region of a silicon substrate 10 is formed by aphotolithography technique. With use of the resist pattern as a mask, B,Ga or In is ion-implanted to the silicon substrate 10. Then, after theresist pattern is removed, a heat diffusion is carried out, and thus aP-type region 11 having a depth of about 1 μm is formed. Next, as can beseen in FIG. 7B, an element isolation region 13 made of a silicon oxidefilm having a thickness of about 600 nm is formed at a predeterminedregion with use of, for example, LOCOS method.

Subsequently, as can be seen in FIG. 7C, a protection oxide film 31 madeof a silicon oxide film having a thickness of about 10 nm is formed, andthen, another ion-implantation is carried out to match the thresholdvalue of the transistor.

After that, as can be seen in FIG. 7D, the protection oxide film 31 isremoved and a gate oxide film 14 having a thickness of about severalnanometers to 10 nm is formed on the exposed surfaces of the siliconsubstrate 10 and the P-type region 11. Further, a WSi_(x)N_(y) film 16having a thickness of about 5 nm is deposited on the entire surface bycarrying out a reactive sputtering in an atmosphere containing Ar and N₂by using a W target.

Next, as can be seen in FIG. 7E, a tungsten (W) film 17 having athickness of about 100 nm is formed by a sputtering in an Ar atmosphereusing a W target, or the CVD method or the like.

Next, a wet etching is carried out using diluted sulfuric acid, so as toremove the native oxide film formed in the surface portion of the W film17. Further, for the removal of the native oxide film, it is possible touse a chemical other than diluted sulfuric acid, or to employ CDE, CMPor the like. Further, the native oxide film may be removed by means ofthe reducing effect which occurs when heated at about 600° C. in anatmosphere of H₂, CO or NH₃, or the like.

Subsequently, as can be seen in FIG. 7F, a silicon nitride film 19having a thickness of about 250 nm is formed by the CVD method or thelike. In the case of the prior art technique, where such a siliconnitride film 19 is formed by the CVD method or the like, under theconditions, a substrate temperature of about 800° C., and a film formingtime of about 30 minutes, whiskers would be generated on the W film 17,which is due to the native oxide film on the W film 17. In thisembodiment, the native oxide film on the W film 17 is already removed,the generation of such needle-shaped products can be effectivelyprevented.

Next, as can be seen in FIG. 7G, a resist pattern 20 for forming adesired gate electrode (wiring) is formed by using the photolithographytechnique. With use of the resist pattern 20 as a mask, the siliconnitride film 19 is etched by the RIE method. Next, the resist pattern 20is removed by using an asher. With use of the silicon nitride film 19 asa mask, the W film 17 and the WSi_(x)N_(y) film 16 are etched by the RIEmethod, and thus a gate electrode or wiring such as shown in FIG. 7H isformed.

Next, a native oxide film existing at this point on the side surface ofthe W film 17 is removed again by any of the above methods, and then asshown in FIG. 7I, a selective oxidization is carried out at 800° C. forabout 30 minutes in an atmosphere of H₂O, H₂ or N₂. With thisoxidization step, only the surfaces of the Si substrate 10 and theP-type region 11 can be oxidized without oxidizing the W film 17.Therefore, the damage to the end of the gate electrode can be lessened,and the concentration of an electrical field is suppressed.

In terms of thermodynamics, a W oxide more easily generates oxo-acidthan W, and therefore unless the removal of an oxide film, the vaporpressure of oxo-acid rapidly increases, and re-crystallization occurs,to generate whiskers. In this example, the native oxide film on the Wfilm 17 is removed, and therefore the growth of whiskers is suppressed.

When a gas is allowed to flow continuously into the chamber during aselective oxidization process, a gas having a low oxo-acid concentrationflows onto the wafer at all times, thus promoting the reaction ofgenerating oxo-acid. In order to avoid this, after the mixture gas ofH₂O, H₂ and N₂, is introduced into the chamber, the gas is sealed in thechamber at an initial stage of the selective oxidization step. With thesealing of the gas, macroscopically, the flow due to the heat convectioncaused by a difference in temperature becomes the dominant portion ofthe entire gas flow occurring within the chamber during the selectiveoxidization process. As a result, the effect of suppressing thegeneration of W oxo-acid can be obtained.

Here, when the H₂O partial pressure, H₂ partial pressure andheat-processing temperature are adjusted properly, the vapor pressure ofW oxo-acid, WH_(x)O_(y), generated can be suppressed at a low level. Forexample, in the case where H₂ partial pressure is set at 2.5×10⁻² Torrand H₂O partial pressure is set at 2.5×10⁻³ Torr, the vapor pressure ofW oxo-acid, WH_(x)O_(y) becomes about 1.5×10⁻¹⁰ Torr. Consequently, thegasification amount of the W film is suppressed to 1 nm or less. Withthe employment of these conditions, the reducing rate of the crosssection of the gate electrode having a width of 0.1 μm, for example,becomes 2% or less.

The value of the reducing rate for the cross section of the gateelectrode, which is directly connected to the gasification amount of theW film 17 in the selective oxidization process, is negligible ascompared to the influences of the other factors which reduce the crosssection of the electrode, including the etching in the lateral directionby RIE when processing the electrode. As the vapor pressure of oxo-acidis decreased, the narrowing of the gate electrode, caused by thegasification of the W film, is suppressed, and therefore it becomespossible to obtain a gate electrode having a designed shape.

Next, as can be seen in FIG. 7J, As is ion-implanted under theconditions: 20 keV and about 5×10¹⁴ cm⁻², thus forming a N⁻-type region24. Then, a silicon nitride film having a thickness of about 50 nm isdeposited by the CVD method, and then the silicon nitride film is etchedby the RIE method, thus forming a side-wall insulation film 25.

Next, as can be seen in FIG. 7K, As is ion-implanted under theconditions: 60 keV and about 7×10¹⁵ cm⁻², thus forming a N⁺-type region27.

Further, as can be seen in FIG. 7L, an interlayer insulation film 31made of a silicon nitride film is deposited, and then a wiring 33 madeof Al is formed in the contact hole 32 connected to the N⁺-type region27.

According to this embodiment, also as to the metal gate of a singlelayer, the vapor pressure of W oxo-acid is controlled during a selectiveoxidization process, so as to obtain a gate electrode of a desired shapeand a resistance value.

It should be noted that the present invention is not limited to theabove-described examples. For example, these examples are directed tothe cases of gate electrodes (wirings) containing W; however in place ofW, some other refractory metal such as MO may be used to obtain asimilar effect. Also, the present invention can be applied to othermetal electrode (wiring) than a gate electrode (wiring).

Apart from the above, the present invention can be remodeled intovarious versions as long as the essence of the invention remains.

As described above, according to the present invention, it is possibleto control the vapor pressure of metal oxo-acid. Consequently, theincrease in the wiring resistance and the degrading of the shape, whichare caused by the narrowing and thinning of the metal wiring, can beprevented, and thus a semiconductor device with a high performance canbe manufactured.

Additional advantages and modifications will readily occurs to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising the steps of: forming a conductive layer containing arefractory metal layer, on a semiconductor substrate; heat-processingthe semiconductor substrate having the conductive layer, in anatmosphere containing water vapor and hydrogen; wherein theheat-processing step is carried out while controlling a vapor pressureof a refractory metal oxo-acid generated on a surface of the refractorymetal layer; and wherein the vapor pressure of the refractory metaloxo-acid is controlled by adjusting at least one parameter selected fromthe group consisting of a ratio between a partial pressure of watervapor and a partial pressure of hydrogen, a temperature of thesubstrate, and a gas flow, wherein the vapor pressure of the refractorymetal oxo-acid is controlled to be 1×10⁻⁶ or less.
 2. A method accordingto claim 1, wherein the heat-processing step is carried out with a gascontaining water vapor and hydrogen sealed in a reaction chamber.
 3. Amethod according to claim 1, further comprising the step of removing anoxide layer formed on a surface of the refractory metal layer, providedbefore the heat-processing step.
 4. A method according to claim 1,wherein the partial pressure of water vapor (PH2O) and the partialpressure of hydrogen (PH2) are controlled to satisfy the followinginequality at a temperature of 800° C.: (PH2O)⁴/(PH2)³<0.17.
 5. A methodaccording to claim 1, wherein the temperature of the substrate duringthe heat-processing step is controlled to be in a range between 700 and950° C.
 6. A method according to claim 1, wherein the refractory metalis one selected from the group consisting of tungsten, molybdenum andtitanium.
 7. A method according to claim 1, wherein the conductive layeris made of the refractory metal layer solely.
 8. A method according toclaim 1, wherein the conductive layer is a laminate member of apolycrystalline silicon layer and the refractory metal layer.
 9. Amethod according to claim 8, wherein a reaction preventing layer isprovided between the polycrystalline silicon layer and the refractorymetal layer.
 10. A method of manufacturing a semiconductor device,comprising the steps of: forming a gate insulating film, on asemiconductor substrate; forming a gate electrode containing arefractory metal layer on the gate insulation film; and heat-processingthe semiconductor substrate in an atmosphere containing water vapor andhydrogen, to lessen a damage caused to a portion of the semiconductorsubstrate, which is located close to an end portion of the gateelectrode; wherein the heat-processing step is carried out whilecontrolling a vapor pressure of a refractory metal oxo-acid generated ona surface of the refractory metal layer; and wherein the vapor pressureof the refractory metal oxo-acid is controlled by adjusting at least oneparameter selected from the group consisting of a ratio between apartial pressure of water vapor and a partial pressure of hydrogen, atemperature of the substrate, and a gas flow, wherein the vapor pressureof the refractory metal oxo-acid is controlled to be 1×10⁻⁶ or less. 11.A method according to claim 10, wherein the heat-processing step iscarried out with a gas containing water vapor and hydrogen sealed in areaction chamber.
 12. A method according to claim 10, further comprisingthe step of removing an oxide layer formed on a surface of therefractory metal layer, provided before the heat-processing step.
 13. Amethod according to claim 10, wherein the partial pressure of watervapor (PH2O) and the partial pressure of hydrogen (PH2) are controlledto satisfy the following inequality at a temperature of 800° C.:(PH2O)⁴/(PH2)³<0.17.
 14. A method according to claim 10, wherein thetemperature of the substrate during the heat-processing step iscontrolled to be in a range between 700 and 950° C.
 15. A methodaccording to claim 10, wherein the refractory metal is one selected fromthe group consisting of tungsten, molybdenum and titanium.
 16. A methodaccording to claim 10, wherein the gate electrode is made of therefractory metal layer solely.
 17. A method according to claim 10,wherein the gate electrode is a laminate member of a polycrystallinesilicon layer and the refractory metal layer.
 18. A method according toclaim 17, wherein a reaction preventing layer is provided between thepolycrystalline silicon layer and the refractory metal layer.